1. Technical Field
The present invention relates to the field of cellular communication networks, and more particularly, to cellular communication base stations.
2. Related Art
Prior to setting forth the background of the related art, it may be helpful to set forth definitions of certain terms that will be used hereinafter.
The term “cellular communication network” as used herein in this application, is defined as any radio frequency (RF) based communication network that is based upon geographical partition of space into cells. Each cell is provided with at least one base station that manages the wireless communication therein. Various cellular communication standards are currently in use while other are being developed. The popular ones are: UMTS, HSPA, GSM, CDMA-2000, TD-SCDMA, LTE and WiMAX.
The term “Universal Mobile Telecommunications System” or “UMTS” as used herein in this application, is one of the third-generation (3G) cell phone technologies, which is also being developed through HSPA into a 4G technology. Currently, the most common form of UMTS uses Wideband Code Division Multiple Access (W-CDMA) as the underlying air interface. W-CDMA is a wideband spread-spectrum mobile air interface that utilizes the direct-sequence spread spectrum method of asynchronous code division multiple access to achieve higher speeds and support more users compared to the implementation of time division multiplexing (TDMA) used by 2G GSM networks.
The term “Femtocell” or “Home Base Station” or “Home Node B” as used herein in this application, is the industry term for a small cellular communication base station, typically designed for use in residential, enterprise or small business environments. The femtocell connects to the service provider's network via broadband Ethernet connection (such as DSL or cable). Current designs typically support two to eight mobile phones simultaneously in a residential setting. A femtocell allows service providers to extend service coverage and capacity indoors, especially where access would otherwise be limited or unavailable. Femtocells provide cellular coverage in a limited area, usually up to few tens or hundreds of meters. The femtocell incorporates the functionality of a typical base station but extends it to allow a simpler, self contained deployment. By way of example, a UMTS femtocell may contain a Node B and RNC with Ethernet connection for backhaul. Femtocells may use different communication standards, including UMTS, HSPA, GSM, CDMA-2000, TD-SCDMA, LTE and WiMAX. In a particular case, the 3G femtocell includes the functionality of the traditional Node-B and RNC, while using the standard 3G/HSPA interface to communicate with the cellular handsets and the internet broadband access for backhauling to the mobile operator.
The femtocells advantages to the operators and to the end user have been much discussed in the public domain. In one aspect of femtocells deployment is the capability of the mobile operator to completely substitute the traditional wireline operator, while convincing the end user to completely switch to cellular handset, i.e. all indoor voice and data communication shall be done using mobile handsets.
One of the femtocell challenges is the requirement for a very low cost solution, suitable for deployment in consumer (residential or SoHo) markets. Nonetheless, in contrast to macrocells, where the BOM cost sensitivity is relatively low, the femtocells cannot use an expensive high frequency oscillators for deriving the clock from backhaul accurate links (e.g. E1/T1). Therefore, to meet this low cost BOM goal, femtocells must use low cost clock sources, such as +/−2 ppm (part per million) TCXO or VCXO oscillators. Nevertheless, the femtocells are base stations, and as such they serve as time and frequency master/references for the handsets camping on them. Accordingly, the femtocells are still required to meet strict frequency accuracy requirements as specified by the different standard or regulatory bodies. For example, 3G femtocells need to have an accuracy of 0.1 ppm, as required, for example, by the standards set by 3rd Generation Partnership Project (3GPP) release 99 through release 7, or 0.25 ppm as required by releases 8 and 9 of the 3GPP.
Due to the aforementioned constraints, the femtocells are required to use other schemes for clock/frequency accuracy, while maintaining the femtocell overall cost low. Several methods have been adopted for frequency correction in femtocells using low cost clock sources: Macrocell sniffing, using Ethernet Time servers with internet clock synchronization protocols, and using of medium cost/medium accuracy oscillator for clocking, for example: TCXO, VCXO, and OCXO.
Several methods have been adopted for frequency correction in femtocells using low cost clock sources: Macrocell sniffing—The femtocells are equipped with the functionality of receiving the downlink channels of high accurate macrocells. The technique consists of periodical receive of the macrocell downlink (e.g. 2G or 3G downlink synchronization and broadcast channels), deriving the frequency and timing from these channels and correcting the femtocell local TCXO/VCXOs. The disadvantages of this technique are: an additional receiver path (the femtos need to receive both uplink and downlink channels over different bands), cost increase and the condition that the femto will be under macrocell coverage. This technique fails in the scenarios of macrocell poor indoor coverage where the femto cannot receive any macrocells downlink channel. In addition, this technique can be used only through the periods where femtocell is not involved in any call—in other words the femto cannot make freq/clock corrections as long as it is involved in any dedicated call; Use Ethernet Time servers with internet clock synchronization protocols, such as IEEE 1588 PTP or NTP. The disadvantage of this technique is the dependency on availability of close internet Grandmasters Time servers, additional load of these protocols on the backhaul internet link and additional hardware in the femtocell for PTP/NTP packets time stamping.
In the case of use of medium cost/medium accuracy—one or multiple TCXO/VCXO/OCXO in combination with the techniques above. In the case of Macrocell sniffing—The femtocells are equipped with the functionality of receiving the downlink channels of high accurate macrocells. The technique consists of periodical receive of the macrocell downlink (e.g. 2G or 3G downlink synchronization and broadcast channels), deriving the frequency and timing from these channels and correcting the femtocell local TCXO/VCXOs. The disadvantages of this technique are: an additional receiver path (the femtocells need to receive both uplink and downlink channels over different bands), cost increase and the condition that the femtocell will be under macrocell coverage. This technique fails in the scenarios of macrocell poor indoor coverage where the femtocell cannot receive any macrocells downlink channel. In addition, this technique can be used only through the periods where femtocell is not involved in any call—in other words the femtocell cannot make freq/clock corrections as long as it is involved in any dedicated call.
Using Ethernet Time servers with internet clock synchronization protocols, like IEEE 1588 PTP or NTP. The disadvantage of this technique is the dependency on availability of close internet Grandmasters Time servers, additional load of these protocols on the backhaul internet link and additional ardware in the femtocell for PTP/NTP packets timestamping.